Multiprotocol Label Switching (MPLS) is utilized to achieve many of the benefits of a circuit-switched network over a packet-switched network. MPLS works by prepending labels to packets and packets are switched according to labels instead of look-up of destination addresses.
One of the fundamental tasks in the MPLS architecture is to exchange labels between label switch routers (LSR) and define the semantics of these labels. LSRs follow a set of procedures, known as label distribution protocol (LDP), to accomplish this task.
LDP peers are two LSRs that use LDP to exchange label information. An LSR might have more than one LDP peer, and it establishes an LDP session with each LDP peer. An LDP session is always bidirectional, which allows both LDP peers to exchange label information.